<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        .box {
            height: 20px;
            width: 20px;
            background-color: aqua;
            border: 1px solid black;
            /*position: absolute;*/
            /*left: 45%;*/
            /*top: 45%;*/
            margin: 0 auto;
        }
    </style>

</head>
<body>
<div class="box">
    小
</div>
</body>
<script type="text/javascript" src="case7.2.3.js"></script>
</html>